The present invention generally relates to golf balls, and more particularly to a golf ball having a solid core and a multi-layer ionomeric cover.
Golf balls traditionally have been categorized in three different groups, namely as one-piece, two-piece and three-piece balls. Conventional two-piece golf balls include a solid resilient core having a cover of a different type of material molded thereon. Three-piece golf balls traditionally have included a liquid or solid center, elastomeric winding around the center, and a molded cover. Solid cores of both two and three-piece balls often are made of polybutadiene and the molded covers generally are made of natural balata, synthetic balata, or ionomeric resins.
Ionomeric resins are polymers containing interchain ionic bonding. As a result of their toughness, durability and flight characteristics, various ionomeric resins sold by E. I. DuPont de Nemours and Company under the trademark xe2x80x9cSurlyn(copyright)xe2x80x9d and by the Exxon Corporation (see U.S. Pat. No. 4,911,451) under the trademark xe2x80x9cEscor(copyright)xe2x80x9d and the trade name xe2x80x9clotekxe2x80x9d, have become the materials of choice for the construction of golf ball covers over the traditional xe2x80x9cbalataxe2x80x9d (transpolyisoprene, natural or synthetic) rubbers. The softer balata covers, although exhibiting enhanced playability properties, lack the durability (cut and abrasion resistance, fatigue endurance, etc.) properties required for repetitive play.
Ionomeric resins are generally ionic copolymers of an olefin, such as ethylene, and a metal salt of an unsaturated carboxylic acid, such as acrylic acid, methacrylic acid or maleic acid. Metal ions, such as sodium or zinc, are used to neutralize some portion of the acidic group in the copolymer, resulting in a thermoplastic elastomer exhibiting enhanced properties, i.e., durability, etc., for golf ball cover construction over balata.
While there are currently more than fifty (50) commercial grades of ionomers available from Exxon and DuPont, with a wide range of properties which vary according to the type and amount of metal cations, molecular weight, composition of the base resin (i.e., relative content of ethylene and methacrylic and/or acrylic acid groups) and additive ingredients such as reinforcement agents, etc., a great deal of research continues in order to develop a golf ball cover composition exhibiting the desired combination of playability properties.
Golf balls are typically described in terms of their size, weight, composition, dimple pattern, compression, hardness, durability, spin rate, and coefficient of restitution (COR). One way to measure the COR of a golf ball is to propel the ball at a given speed against a hard massive surface, and to measure its incoming and outgoing velocity. The COR is the ratio of the outgoing velocity to the incoming velocity and is expressed as a decimal between zero and one.
There is no United States Golf Association limit on the COR of a golf ball but the initial velocity of the golf ball must not exceed 250xc2x15 ft/second. As a result, the industry goal for initial velocity is 255 ft/second, and the industry strives to maximize the COR without violating this limit.
U.S. Pat. Nos. 4,431,193 and 4,919,434 disclose multi-layer golf balls. U.S. Pat. No. 4,431,193 discloses a multi-layer ball with a hard inner cover layer and a soft outer cover layer. U.S. Pat. No. 4,919,434 discloses a golf ball with a 0.4-2.2 mm thick cover made from two thermoplastic cover layers.
An object of the present invention is to provide a golf ball having a soft compression while maintaining good durability and a high coefficient of restitution.
Another object of the invention is to provide a golf ball having a good coefficient of restitution as compared to other golf balls having a similar compression and surface hardness.
Yet another object of the invention is to provide a soft golf ball having a moderate spin rate and good carrying distance while maintaining a relatively soft compression.
Another object of the invention is to provide a method of making a golf ball having the properties described above.
The invention in a preferred form is a golf ball comprising a core, an inner cover layer comprising an ionomer and having a Shore D hardness of about 65 or less (more preferably, 56-65), and an outer cover layer comprising an ionomer and having a Shore D hardness which is greater than the Shore D hardness of the inner cover layer and is at least about 60 (more preferably 65-71). The sum of the thickness of the inner cover layer and the thickness of the outer cover layer is at least 0.090 inches.
The core is preferably comprised of a high Mooney viscosity polybutadiene resin. More preferably, the polybutadiene resin is a neodymium catalyzed butadiene rubber.
The golf ball of the invention preferably has a COR of at least 0.780, more preferably at least 0.790, and most preferably at least about 0.800. The golf ball has a weight in the range of 43.0-46.0 g. Preferably, the ratio of the diameter of the ball to the sum of the thickness of the inner cover layer and outer layer is less than 19:1 and more preferably less than about 18:1. The PGA compression of the golf ball according to a preferred embodiment of the invention is no more than about 80, more preferably no more than about 70 and even more preferably no more than about 65.
Another preferred form of the invention is a method for making a multi-layer golf ball. The method comprises the steps of (1) obtaining a golf ball core, (2) forming an inner cover layer over the core, the inner cover layer comprising an ionomer and having a Shore D hardness of about 60 or less and (3) forming an outer cover layer over the inner cover layer, the outer cover layer having a Shore D hardness of at least about 60 and being harder than the inner cover layer. The sum of the thickness of the inner cover layer and the thickness of the outer cover layer is at least 0.090 inches.
The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others and the article possessing the features, properties, and the relation of elements exemplified in the following detailed disclosure.